Could you try opening the files with Office Online using another web browser? Also, please try using an incognito/private window and clearing the browser's cache to see if there's any difference in the behaviour.
Can you also clarify for me if, when you open the file on your desktop, you have downloaded the file directly from the website or from the desktop app's Dropbox folder on your computer?
Finally, when opening the file with Office Online, do you see any error messages at all anywhere or do you just see a blank doc? Would you mind sharing a screenshot of this (excluding any personal info)?
Let me know what you find and we'll go from there - thanks!
If the issue seems to be isolated to Chrome, then it might be worth checking if you can access the files in an incognito window of Chrome. If you can, then it suggests that a browser extension is the culprit, and you can try disabling your extensions one at a time until the issue is resolved to isolate which one it is.
Otherwise I can suggest, if you haven't already, to clear the cache of your Chrome browser and even just check that there's no pending updates that might fix the issue.